1. Field of the Invention
The present invention relates to a casting device for discharging a viscoelastic fluid, a solution casting apparatus, and a solution casting method, and especially, a casting device for discharging a dope which contains a polymer as a raw material of a film, and a solution casting apparatus for and a solution casting method of producing a film from the dope.
2. Description Related to the Prior Art
A polymer film (hereinafter, film) is variously used as an optical functional film, since it being excellent in the excellent transparency and flexibility, and further since the film thickness can be decreased. Among the polymer film, there is a cellulose acylate film formed from cellulose acylate, and especially, a cellulose triacetate (hereinafter TAC) is used among the cellulose acylate, so as to form a TAC film. The TAC film is used as a film base of a film material, such as a photosensitive material, since having strength and inflammability. Note that, among the TAC, the averaged acetylation degree is preferably in the range of 57.5% to 62.5%. Further, the TAC film is excellent in optical isotropy, and therefore used as a protective film for a polarizing filter, an optical compensation film (for example, a wide view film and the like) in a liquid crystal display whose market becomes larger in recent years.
As a method of producing the film, there are a melt extrusion method and a solution casting method. In the melt extrusion method, the polymer is heated and molten, and then the molten polymer is extruded from an extruder to form the film. The melt extrusion method has merits in the high productivity and the low cost for furnishing the equipments. However, in the melt extrusion method, it is difficult to control the film thickness finely, and since stripes (named die line) is formed on the film, the produced film hardly has a high quality adequate to the optical film. Otherwise, in the solution casting method, a polymer is dissolved to a solvent, and thus a dope as a polymer solution is prepared. Then the dope is cast from a casting die onto a support so as to form a casting film. When the casting film has a self-supporting property, the casting film is peeled as a wet film from the support. Thereafter, in a tenter dryer, while both side edge portions of the wet film is clipped, The wet film is stretched and a relaxation thereof is made. At the same time, the wet film is dried. After the drying is made enough, the wet film is wound up as the film. In the solution casting method, the produced film is more excellent in the optical isotropy and the thickness uniformity than, and contains less foreign material than in the melt extrusion method. Therefore, the solution casting method is well known as a preferable method of producing the film, especially the optical film.
In following, outlines of a casting process in the solution casting method will be explained. As shown in FIG. 12 illustrating a discharging process, a casting die 300 as the casting device discharges a dope 301 onto a running support 302. For the casting, the dope 301 is spread on the support 302 by the running of the support 302 in a running direction X1. Thus a casting film 303 is formed.
The casting die 300 includes a dope outlet 300a through which the dope 301 is discharged. As shown in FIG. 13, the casting die 300 is provided with lip plates 310, 311, side plates 312, 313, and inner deckle plates 314, 315, while the lip plates 310, 311 and the inner deckle plates 314, 315 are positioned so as to form the dope outlet 300a and respectively have contact face 310a, 311a, 314a, 315a for contacting the dope 301.
When the casting of the casting dope 301 onto the support 302 is made, the casting dope 301 is discharged through the dope outlet 30a from the casting die 300, so as to form a dope bead between the outlet 300a and the support 302. However, the retaining of the casting dope 301 easily occurs near both sides of the outlet 300a on the inner deckle plates 314, 315, and the casting dope 301 in retaining is easily dried to form a skinning. Then the skinning grows to have an icicle form, which disturbs the flow of the casting dope 301 through the outlet 301. Thus, the formation of the casting film 303 and a bead of the discharged casting dope is inhibited. Therefore, in the case that the skinning occurs, the casting is removed by decreasing the casting speed. In this case, it is necessary to make the casting speed lower such that the casting film and the bead may not be cut off. As a result, the skinning in the casting process causes the extremely low productivity.
Conventionally, as the method of preventing the skinning, there are a method of providing a solvent for preventing the dry of the casting dope while the dope is dissolvable to the solvent, a method of preventing the retaining of the casting dope near the outlet. Japanese Patent Publication No. 2,687,260 and Japanese Patent Laid-Open Publication No. 2002-337173 disclose methods of preventing the skinning, which are hereinafter named a flowing down method. In these methods, while the casting dope is discharged from the casting outlet, a solvent (dichloromethylene and the like) is flown down onto both side edges of the discharged dope, each of which is disposed slightly inside from outlet edges of the casting die. In Japanese Patent-Laid Open Publication No. 2002-103361, according to the sectional shape of both side edges of a lip end of the casting die, the skinning is prevented while an angle between a lip faces and a lip side face is controlled to at least 120°.
However, in the casting process, the wind speed near the outlet 300a and the supply rate of the solvent are hardly controlled to the predetermined values. In the methods of the publications No. 2687260 and No. 2002-337173, if the controls of the wind speed and the supply rate are not made adequately, the solvent flowing down disperses over the support, which causes the planarity defect of the film, and the solvent is supplied excessively, which causes the adhesion of the foreign materials to the support. If the adhesion of the foreign materials occurs, the support must be cleansed, and therefore the productivity becomes lower. Further, in accordance with the extremely growth of the demand of the liquid crystal display, it is required to progress the high technology of the solution casting method. However, if the production speed is made higher, the controls of the wind speed and the temperature near the outlet 300a are more hardly controlled. Therefore, it is difficult to apply the above flowing down methods to the solution casting method. Further, in the method of the publication No. 2002-103361, the outlet is hardly formed at such a processing accuracy that the skinning may be prevented. Therefore, the effect of preventing the skinning is not expected enough in the conventional art.
In the researches and keen examinations, the inventor found that the retaining causing the skinning is prevented if the lower ends of the lip plates 310, 311 and the inner deckle plates 314, 315 are on an imaginary face, namely if the displacement among the lip plates 310, 311 and the inner deckle plates 314, 315 in a flow-out direction (referred to as a discharging direction) of the dope 301 is in the predetermined value.